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Section 8
Catalytic Converter Diagnosis
Front O2 Sensor
Sub O2 Sensor
O2 Sensor waveform with ordinary catalyst
Learning Objectives:
1.Determine the condition of the catalytic converter based on data and
engine symptoms, and determine appropriate repair.
Engine Control Systems II - Course 874
Section 8
Catalytic Converter Diagnosis
Catalytic Converter System
Wall Thickness
HC
CO
NO
x
O
2
H
2
O
CO
2
N
2
Fig. 8-1
TL874f801
Overview
Catalytic converter failures generally fall in the category of physical
damage or catalyst failure. Physical damage usually can be visually
identified-cracks, dents, etc. Internally, the structure can be cracked,
broken, or melted. Where high heat may have lead to catalyst failure,
the engine and related systems need to be thoroughly checked.
Catalyst performance before OBD II was determined differently in many
states by test equipment. OBD II systems can determine catalyst
performance. Performance deteriorates in many cases when the catalyst
becomes coated with foreign materials. Contaminated fuel, sealants, or
coolant can all affect catalyst performance.
The sub (S2) O2 sensor is used to adjust the oxygen level in the
catalytic converter to achieve the best catalytic converter efficiency
possible. As a catalytic converter deteriorates, its ability to store oxygen
is also reduced. During conversion, the stored oxygen is rapidly
depleted. The sub O2 sensor detects this and, within a very limited
range, the ECM will reduce the amount of fuel injected providing more
oxygen to converter. Oxygen levels build up, driving the O2 signal
downward. At a predetermined point, fuel control will return to
stoichiometric A/F Ratio. When this happens, oxygen again will be
depleted driving the sub O2 signal upward, and the cycle will repeat
itself. The rate at which this cycle repeats depends on how much of the
catalyst has deteriorated, engine load, and the amount of correction as
determined by the ECM's fuel control programming.
Engine Control Systems II - Course 874
8-1
Section 8
NOTE
• Fuel control by the sub O2 sensor is a very fine adjustment. Its
affect on fuel trim and driveability is extremely limited.
• The cycling of the sub O2 sensor signal is NOT to produce an
averaging of exhaust gases to achieve stoichiometry.
• The affect on the A/F sensor is practically impossible to see on the
Diagnostic Tester because of the slow data rate, and the speed of
fuel correction. The sub O2 response is slower because of the
catalytic action and the fact it is an O2 sensor.
• The ECM on an A/F sensor equipped engine does try hold the A/F
sensor signal at a constant level, but with these sensors there is
hysteresis taking place. In other words there is some variation in the
signal. This should not be confused with the cycling action found
with fuel control programs used with O2 sensor equipped engines.
• The specification for the sub O2 sensor signal frequency to set a
DTC is going to vary with the certification level of the vehicle- LEV,
ULEV, SULEV.
It is important to determine why the catalytic converter failed before
replacing it.
NOTE
Catalyst deterioration is determined by monitoring the amount of
oxygen in the exhaust stream after the catalytic converter. The actual
level of emission gases in the tailpipe is not monitored.
Catalytic
Converter
Monitor
The diagnostic system measures the oxygen storage capacity of the
catalyst. This is based on the correlation between catalyst conversion
efficiency and oxygen storage capacity. Catalyst efficiency is monitored
by comparing the pre-catalyst O2 or A/F sensor output signal with the
signal received from the post-catalyst O2 sensor. The ECM uses voltage
variations between these sensors to measure the catalyst performance.
When the converter is operating properly, the post-catalyst sensor is
significantly less active than the pre-catalyst sensor. This is because the
converter stores and releases oxygen as needed during its reduction and
oxidation processes, thus the post-catalyst sensor is exposed to exhaust
gases with very little variation in oxygen levels.
8-2
TOYOTA Technical Training
Catalytic Converter Diagnosis
O2 Waveform
Normal Operation
Catalyst efficiency is
monitored by comparing
the pre-catalyst O2 or
A/F sensor output signal
with the signal received
from the post-catalyst O2
sensor.
Front O2 Sensor
Sub O2 Sensor
O2 Sensor waveform with ordinary catalyst
Fig. 8-2
TL874f802
Main O2 vs.
Sub O2 Signal
The sub-sensor
signal will make slow
transitions in voltage,
however, they occur over
long periods of time
(several seconds). The
sub-sensor signal from a
catalytic converter that is
not functioning properly
will have a pattern with a
frequency and amplitude
similar to the main O2
sensor.
Fig. 8-3
TL874f803
Engine Control Systems II - Course 874
8-3
Section 8
A/F Sensor vs.
Sub O2 Signal
Waveform of
A/F Sensor
Normal Catalyst
Waveform of Heated O2
Sensor behind Catalyst
Catalyst efficiency is
monitored by comparing
the pre-catalyst A/F
sensor output signal with
the signal received from
the post-catalyst O2
sensor. The A/F sensor
signal does not switch in
the same manner as the
O2 sensor, therefore
the signal will reflect
less activity. When the
vehicle is accelerated
under load with a bad
catalytic converter, there
will be excess oxygen
passing through the
converter lowering the
sub O2 sensor signal.
Abnormal Catalyst
Fig. 8-4
TL874f804
NOTE
After the engine and catalyst are warmed up and the conditions listed
are met, the ECM will run the catalyst monitor. Catalyst warm-up is
determined by a calculation in the ECM’s internal programming. Engine
load, engine coolant temperature, and time are the primary factors used
to determine catalyst temperature.
8-4
TOYOTA Technical Training
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